The well-known Wurster apparatus and process for coating particulate material in a fluidized bed relies upon an air stream for moving and suspending the product (such as granules, beads, pellets, tablets) during coating, layerings, and drying. The Wurster apparatus generally includes a container with a cylindrical partition extending upwardly therein, and with a perforated plate or screen at the lower end thereof to define a bottom wall for the particles. The partition is spaced above the perforated plate. The area within the cylindrical partition defines the upbed of the container, while the area outside the partition defines the downbed of the container. The perforated plate includes an area of large perforations and a greater percentage of perforated open area through which air flows into the upbed at an increased velocity, and an area of perforations with a lower percentage of open area through which air flows into the downbed at a decreased velocity. The higher velocity air in the upbed area transports the particles for coating, layering, and drying of a coating solution sprayed from a spray nozzle extending upwardly through the perforated plate and into the upbed area. The particles then encounter the lower velocity air in the expansion chamber above the partition. When the air velocity is insufficient to support the product, the particles fall into the downbed area for re-entry into the higher velocity air, such that a cycle of coating in the upbed area and drying in the downbed area is achieved. Various forms of the Wurster apparatus and process are disclosed in U.S. Pat. Nos. 2,648,609, 2,799,241, 3,089,824, 3,196,827, 3,207,824, and 3,253,944.
In the conventional Wurster apparatus, the material passes closely to the spray nozzle. The close proximity of the material to the spray nozzle prevents complete development of the spray pattern, produces incomplete atomization, and can result in agglomeration of the material at modest spray rates. Uneven or non-uniform coating of the material also results from the close passage of the material to the spray nozzle as the material is carried upwardly in the upbed area of the Wurster container.
Therefore, a primary objective of the present invention is the provision of an improved Wurster having an air diverter for improving the coating process.
Another objective of the present invention is the provision of an air diverter for a Wurster apparatus that allows more complete development of the spray pattern and more complete atomization of the spray solution.
A further objective of the present invention is the provision of an improved Wurster, which eliminates or minimizes agglomeration of the material as it is sprayed.
Another objective of the present invention is the provision of a sleeve mounted on the spray nozzle and connected to a gas or air line to supply air through holes in the sleeves and to expel the air in a radial direction adjacent the nozzle so as to move material laterally away from the nozzle during the spraying operation.
Another objective of the present invention is the provision of an improved Wurster wherein the volume of air and air pressure passing through the diverter can be monitored and controlled to enhance performance of the Wurster as the product grows in size throughout the coating process.
A further objective of the present invention is the provision of an improved Wurster process having a primary flow of air to carry the material in a cyclical path through the upbed and downbed areas of the Wurster container, and a secondary flow of air adjacent the spray nozzle to move the product away from the nozzle during spraying.
Another objective of the invention is the provision of an improved Wurster having increased spray rate due to the improved product flow characteristics within the spray zone.
A further objective of the present invention is the provision of an air diverter for a Wurster having various shaped sleeves and/or multiple columns.
These and other objectives will become apparent from the following description of the invention.
The improved Wurster apparatus of the present invention includes an air diverter for moving products away from the spray nozzle during the spraying process. More particularly, the air diverter comprises a sleeve extending around the spray nozzle body and having a plurality of holes adjacent the spray nozzle. The sleeve is operatively connected to a pressurized air line which forces air outwardly through the holes in the sleeve in a circumferential direction. The force generated by the velocity of the expelled air or gas pushes the material away from the nozzle, while the volume of the expelled air or gas prevents the material from collapsing back inwardly around the nozzle until the material is past the spray zone. The air diverter increases the effective area of the spray zone to allow the spray pattern to develop more fully, and to allow higher spray rates with little or no agglomeration of the product.
In the improved Wurster process of the present invention, a primary air flow is directed into the upbed and downbed areas of the Wurster container in a conventional manner so as to cycle the product upwardly through the upbed area for coating and/or layering, and downwardly through the downbed area for re-introduction to the spray zone. The process improvement includes a secondary air flow directed radially outwardly adjacent the spray nozzle to move and maintain the product in a spaced relation to the spray nozzle for more fully developed spray pattern, increased spray rates, and elimination or minimization of agglomeration of the coated material.